Any printable color can be made up by mixing together appropriate concentrations of cyan and magenta and yellow inks. Black ink is very often used in addition in order to provide denser blacks, and single inking for black text. Other inks in addition, such as red and green or light cyan, light magenta, light yellow are also used on some printers.
The accuracy of the resultant color depends the accuracy of the concentration of each of the inks making it up. For a given set of inks, the color can be defined by the amounts of each ink being used. Controlling the amount of each ink that is put down is critical for the accurate reproduction of color images.
Laying the inks all over the paper with varying thickness of inks is one way to create variable color. In this process the amount of ink actually applied is critical to the color. This is said to be continuous tone. Chemical photographic prints work like this with cyan, magenta and yellow dyes.
Another way to create an effectively variable concentration for each ink is halftone screening. In this technique a layer of ink of uniform thickness is put on the paper with a fine dot pattern. The halftone dots are of variable size (or may be of variable frequency), and the resultant viewed color (averaged over a small area) depends on the proportion by area of the paper that is covered by that ink. This is known as the dot percentage (dot %). Zero percent (0%) dot is white paper. One hundred percent (100%) dot is solid ink.
Computer printers for home and office use have only in recent years become capable of imaging photographs. To do so, such printers use continuous tone or, more often, halftone techniques or a mix of the two techniques. Color from such printers has generally not been very accurate, but this is now changing. When such printersxe2x80x94or any printers, for that matterxe2x80x94use halftones, the printing process is subject to varying dot gain, which must be controlled for good results.
Color management systems using color profiles are now commonly available on personal computer systems. A color management system is a collection of color management software tools used to try to make color device-independent. Ideally, the colors on a computer monitor accurately represent the colors in a scanned image, or the colors on a printed page. A color profile describes a complete table of all the colors that a device can input or reproduce.
The chemistry, concentration and colorimetry of printing inks is controlled very accurately in the manufacturing process. Essentially all printers of a printer family use identical ink. By design, the color of an ink may differ from a pure color. Generally, for example, cyan ink is pure cyan plus some (e.g., 30%) pure magenta plus a trace (e.g., 5%) of pure yellow; that is, the ink is actually to the blue side of cyan. Similarly, magenta ink is generally pure magenta plus some (e.g., 30%) pure yellow; that is, it is actually to the red side of magenta. Generally, yellow ink is pure yellow with very little impurity; and black ink is pure black.
In this specification, the part of a printer that delivers ink to paper will be referred to as a xe2x80x98print headxe2x80x99. The print head may be replaceable, and it may be made up of multiple, separately-replaceable units.
The amount of ink that is put down in each pel (a pel is a printing element or dot) can vary from one print head to another by perhaps as much as 20%. This alters the average color of any small area both because the larger volume of ink spreads sideways creating a larger area pel, and may within the pel form a thicker layer of ink of greater density. The halftone dots used in printing an image are generally but not necessarily made from more than one printer pel. Some print heads can create pels of several sizes.
If a particular instance of a printing system puts down more ink than is intended, the system is said to be exhibiting dot gain. For example, if 60% of a paper area is covered in ink where 50% coverage was requested, the system is said to be exhibiting 10% dot gain. Strictly speaking, this terminology only applies to halftone reproduction. However, it can be applied to continuous tone reproduction by measuring the density of the ink component of a color and calculating the equivalent dot % to produce the same color.
Dot gain is not constant. Dot gain can change when a print head is replaced and as a print head ages. Different paper types can also exhibit different dot gain.
Electronic printing devices generally exhibit nonlinear output behavior. A transfer function adjusts the values of color components to compensate for nonlinear response in an output device and in the human eye. Each component of a device color space is intended to represent the perceived lightness or intensity of that color component in proportion to the component""s numeric value. Many devices do not actually behave this way, however; the purpose of a transfer function is to compensate for the device""s actual behavior.
A transfer function is often thought of as a curve and may therefore be referred to as a transfer curve or halftone curve. A transfer function can be implemented simply as a one-dimensional look-up-table of output values for input values. The output values define the shape of a curve which compensates for the output characteristic of the printer. Transfer functions are described in reference works such as Adobe Systems Incorporated, PostScript(copyright) LANGUAGE REFERENCE third edition, section 7.3, Transfer Functions, pages 478 et seq. An appropriately defined transfer curve can compensate for dot gain in a printer.
The prior art includes at least five different approaches to achieving consistency in the color which printers reproduce. In one approach, printer control software is provided that includes a simple way of making the output lighter or darker, by turning a metaphorical dial, for example. In a second approach, the user prints a picture that the user is familiar with and adjusts parameters of some color space, repeatedly printing the picture until satisfied with the result. In a third approach, a reference image is printed and measured using a spectrophotometer or densitometer. In a fourth approach, the printer incorporates a densitometer and is capable of recalibrating itself. The fifth approach is a xe2x80x98grey balancexe2x80x99 method, where a gray patch of color is compared to a mix of cyan, magenta and yellow ink that gives a grey color. The user picks from a series of CMY (cyan, magenta, yellow) patches the patch which looks most like a reference gray patch.
The problem of undesirable dot gain can be corrected by increasing or reducing the quantity of ink used to print a required color. To determine the amount of correction required for each of the inks, a visual calibration assessment chart is printed on the printer. For each of one or more colors of ink in the print head, the user is shown a series of shaded patches of that ink. The user picks which shaded patch looks most like a fixed invariant shaded patch of the same color ink. The invariant patch is a color reference printed by the printer surrounding each instance of the differently shaded patches. The size of the correction is determined from the position of the shaded patch within the series.
The invariant patch is a stable reference created by printing a very coarse halftone screen. Because the dots of the invariant patch are large, the dot gain due to a change of print head is much smaller than that of the series of shaded patches, which are printed with normal halftoning. The dot gain of the normally halftoned shaded patches is assessed by visual comparison with the fixed invariant shaded patch.
In general, in one aspect, the invention provides techniques that can be implemented as methods, systems, or apparatus, including computer program apparatus, for calibrating a printer. The techniques include printing a color reference for a reference tint on a substrate, the color reference being a coarse halftone screen printed using a single colorant; printing a sample for each of multiple tints of the colorant, each sample being printed using a normal mode of the printer for printing the sample; receiving from a user who has viewed the printed substrate a calibration input based on the user""s comparison of the printed samples to the printed color reference; and modifying the transfer curve of the printer according to the calibration input.
In general, in another aspect, the invention provides techniques for printing a color sample and a color reference both based on one colorant for visual comparison by a person. These techniques include printing the sample and the reference simultaneously on a substrate, the sample surrounded by the reference, the reference being printed as a coarse halftone screen of the colorant and the sample being printed as a tint of the colorant in a normal mode of operating the printer.
In general, in another aspect, the invention provides techniques for making a stable invariant color reference on a computer printer. These techniques include printing a coarse screen of full ink on a computer printer to make the stable invariant color reference on the computer printer.
Advantages that can be seen in implementations of the invention include one or more of the following.
The invention will work for printers which use either halftone or continuous tone, because a continuous tone device can print a coarsely screened halftone.
The invention can compensate for any change of dot gain, regardless of the reason for the change. For example, the change might occur because a different instance of the same printer model is being used, because a print head was changed, because a print head has aged, because different paper or other media are being used, or because a change in temperature or humidity has affected the appearance or behavior of the ink or medium.
The invention enables different printers to reproduce the same color with little or no perceivable difference. The invention requires no use of electronic or optical measuring devices. The invention reduces the problem of visual color matching by simplifying the choices that the user has to make.